Method and apparatus for a zero voltage processor sleep state

ABSTRACT

Embodiments of the invention relate to a method and apparatus for a zero voltage processor sleep state. A voltage regulator may be coupled to a processor to provide an operating voltage to the processor. During a transition to a zero voltage power management state for the processor, the operational voltage applied to the processor by the voltage regulator may be reduced to approximately zero while an external voltage is continuously applied to a portion of the processor to save state variables of the processor during the zero voltage management power state.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No. 13/149,873, filed May 31, 2011, now U.S. Pat. No. 8,707,066, issued Apr. 22, 2014, which is a continuation of U.S. application Ser. No. 11/937,139, filed Nov. 8, 2007, now U.S. Pat. No. 7,953,993, issued May 31, 2011, which is a continuation-in-part of U.S. application Ser. No. 11/323,254, filed Dec. 30, 2005, now U.S. Pat. No. 7,664,970, issued Feb. 16, 2010, all of which are hereby incorporated by reference in their entirety into this application.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 10/931,565 filed Aug. 31, 2004 by inventors Kurts et al., now U.S. Pat. No. 7,363,523, issued Apr. 22, 2008; U.S. application Ser. No. 10/934,034 filed Sep. 3, 2004 by inventors Naveh et al., now U.S. Pat. No. 7,451,333, issued Nov. 11, 2008; U.S. application Ser. No. 11/024,538 filed Dec. 28, 2004 by inventors Naveh et al.; U.S. application Ser. No. 10/899,674 filed Jul. 27, 2004 by inventors Naveh et al., now U.S. Pat. No. 7,966,511, issued Jun. 21, 2011; and U.S. patent application Ser. No. 11/323,259, filed Dec. 30, 2005.

BACKGROUND

1. Field

Embodiments of the invention relate to the field of electronic systems and power management. More particularly, embodiments of the invention relate to a method and apparatus for a zero voltage processor sleep state.

2. Description of Related Art

As the trend toward advanced microprocessors, e.g. central processing units (CPUs), with more transistors and higher frequencies continues to grow, computer designers and manufacturers are often faced with corresponding increases in power and energy consumption. Particularly in mobile devices, increased power consumption can lead to overheating, which may negatively affect performance, and can significantly reduce battery life. Because batteries typically have a limited capacity, running the processor of a mobile device more than necessary could drain the capacity more quickly than desired.

Thus, power consumption continues to be an important issue for mobile devices including laptop computers, wireless handsets, personal digital assistants, etc. In today's mobile devices, for example, to address power dissipation concerns, certain components may be placed into lower power sleep states based on reduced activity or demand.

For one approach, an operating system may support a built-in power management software interface such as Advanced Configuration and Power Interface (ACPI) (e.g. Advanced Configuration and Power Interface, Ver. x285, June 2004). ACPI describes a power management policy including various “C states” that may be supported by processors and/or chipsets. For this policy, C0 is defined as the Run Time state in which the processor operates at high voltage and high frequency. C1 is defined as the Auto HALT state in which the core clock is stopped internally. C2 is defined as the Stop Clock state in which the core clock is stopped externally. C3 is defined as a Deep Sleep state in which all processor clocks are shut down, and C4 is defined as a Deeper Sleep state in which all processor clocks are stopped and the processor voltage is reduced to a lower data retention point. Various additional deeper sleep power states C5 . . . Cn have also been proposed. These additional power states are characterized by equivalent semantics of the C1 through C4 powers states, but with different entry/exit latencies and power savings.

In operation, to enter the deeper sleep states, ACPI may detect a time slot in which there are no new or pending interrupts to the mobile processor. The ACPI policy then uses an input/output (I/O) controller or other chipset features to place the mobile processor into the deeper sleep states.

Once the processor is placed into the deeper sleep state, a break event or interrupt from the operating system or another source may be sent to the chipset, and the chipset will then allow the processor to exit the deeper sleep state. The ability to transition between various power management states, including deeper sleep states, may enable power dissipation to be reduced and battery life to be increased.

Currently, entry into deeper sleep states is done by referencing an external voltage reference in a processor voltage regulator circuit and regulating to this reference voltage whenever a platform “Deeper Sleep” signal such as a DPRSLPVR signal or other similar signal is asserted by the I/O controller or other integrated circuit. The voltage regulator then transitions from a first voltage to a second lower voltage associated with the deeper sleep state. Upon exiting the deeper sleep state, a voltage transition in the other direction takes place with a similar specified time window.

As previously noted, obtaining low power sleep states is important to achieving better battery life in mobile devices. The mobile device market is a fiercely competitive product space and one of the key areas for advancement in this space is low-power solutions to preserve battery life.

Unfortunately, existing deeper sleep states for processors in mobile devices still burn a non-neglible amount of power because voltage is still required to be applied to the processor and cannot be completely powered off.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a process to transition into and transition out of a zero voltage power management state for a processor, according to one embodiment of the present invention.

FIGS. 2A and 2B are block diagrams of an exemplary system that may be utilized to implement the zero voltage power management state approach, according to one embodiment of the present invention.

FIG. 3 is a block diagram illustrating an example of a dedicated sleep state SRAM cache and an SRAM interface, according to one embodiment of the present invention.

FIG. 4 is a flow diagram illustrating a process that may be utilized to enter into a zero voltage processor sleep state, according to one embodiment of the present invention.

FIG. 5 is a block diagram illustrating a process that may be utilized to exit out of the zero voltage processor sleep state, according to one embodiment of the invention.

FIGS. 6A and 6B are block diagrams of an exemplary system that may be utilized to implement the zero voltage power management state approach, according to another embodiment of the present invention.

FIG. 7 is a flow diagram illustrating a process to transition into and transition out of the zero voltage power management state for a processor, according to another embodiment of the present invention.

FIG. 8 is a block diagram of an exemplary system that may be utilized to switch contexts between processor cores, according to another embodiment of the present invention.

DESCRIPTION

In the following description, the various embodiments of the invention will be described in detail. However, such details are included to facilitate understanding of the invention and to describe exemplary embodiments for employing the invention. Such details should not be used to limit the invention to the particular embodiments described because other variations and embodiments are possible while staying within the scope of the invention. Furthermore, although numerous details are set forth in order to provide a thorough understanding of the embodiments of the invention, it will be apparent to one skilled in the art that these specific details are not required in order to practice the embodiments of the invention.

In the following description, particular components, circuits, state diagrams, software modules, systems, timings, etc. are described for purposes of illustration. It will be appreciated, however, that other embodiments are applicable to other types of components, circuits, state diagrams, software modules, systems, and/or timings, for example.

With reference to FIG. 1, in one embodiment, an integrated circuit device such as a processor, for example, initiates a transition to a zero voltage power management state at block 105. The zero voltage power management state may be, for example, a Deeper Sleep state in accordance with the Advanced Configuration and Power Interface (ACPI) Specification, Revision 2.0a dated Mar. 31, 2002 (and published by Compaq Computer Corporation, Intel Corporation, Microsoft Corporation, Phoenix Technologies Ltd., and Toshiba Corporation). During this transition, the critical state of the processor is saved (block 110). The critical state of the processor includes state variables associated with the architectural, micro-architectural, debug state, and/or similar state variables associated with that processor. The operating voltage of the processor is subsequently reduced to approximately zero such that the processor is in a very deep sleep state that has very low power consumption characteristics (block 115). Hereinafter reference to the state or critical state of the processor or CPU will be meant to include state variables associated with the processor or CPU.

Subsequently, in response to receiving a request to exit the zero voltage power management state, the processor exits the zero voltage power management at a higher reference operating voltage at block 120. The critical state variables associated with the processor are also restored (block 125). It should be noted that for some embodiments, the reference operating voltage may be a minimum active state operating voltage, for example.

Further details of this and other embodiments are provided in the description that follows.

Embodiments of the invention may be implemented in one or a combination of hardware, firmware, and software. Embodiments of the invention may also be implemented in whole or in part as instructions stored on a machine-readable medium, which may be read and executed by at least one processor to perform the operations described herein. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others.

FIGS. 2A and 2B are block diagrams of an exemplary system 200 that may implement the zero voltage power management state transition approach of one or more embodiments. The system 200 may be a notebook or laptop computer system, or may be any different type of mobile electronic system such as a mobile device, personal digital assistant, wireless telephone/handset or may even be a non-mobile system such as a desktop or enterprise computing system. Other types of electronic systems are also within the scope of various embodiments.

The system 200 includes a processor 205, a platform-level clock generator 211, a voltage regulator 212 coupled to the processor 205, a memory control hub 215 coupled to the processor 205 over a bus 217, a memory 220 which may comprise one or more of random access memory (RAM), flash memory and/or another type of memory, an input/output (I/O) control hub 225 coupled to the memory control hub 215 over a bus 227, and a mass storage device 230 coupled to the I/O control hub 225 over a bus 232. Although, system 200, in one embodiment, may be a mobile device with the subsystems described, it should be appreciated that system 200 may be a different type of mobile device or a non-mobile device, with more or less than the subsystems described.

In one embodiment, the processor 205 may be an Intel® architecture microprocessor such as, for example, a follow-on processor to the Intel Pentium® M processor including one or more processing cores (e.g. 320 and 322) and at least one execution unit 310 to process instructions. For such embodiments, the processor 205 may include Intel SpeedStep® technology or another power management-related technology that provides for two or more voltage/frequency operating points. An associated clock/power management unit 350 may be included in the processor 205 to control transitions between two or more of the voltage/frequency pairs.

In other embodiments, the processor 205 may be a different type of processor such as a digital signal processor, an embedded processor, or a microprocessor from a different source.

Further, processor 205 may include a dedicated cache memory 340 (e.g. synchronous random access memory (SRAM)) that may be used to store the processor's critical state variables when the processor enters the zero-voltage sleep state, as will be described. Cache memories may be built into the processor's chip or packaged within the same housing as the processor chip.

Where Intel SpeedStep® technology or another type of power management technology is included on the processor 205, the available voltage/frequency pairs associated with the technology include a minimum voltage/frequency pair corresponding to a minimum active mode operating voltage and a minimum operating frequency associated with the processor 205 for a fully functional operational mode. These may be referred to herein as the minimum operating voltage and minimum operating frequency or minimum active mode operating voltage and frequency, respectively. Similarly, a maximum operating voltage and frequency may be defined. Other available voltage frequency pairs may be referred to as operating voltage/frequency pairs or simply other voltage/frequency or frequency/voltage pairs.

Zero voltage entry/exit logic 354 may also be included in processor 205, either within or outside of the power management logic 350, to control entry into and exit from the zero voltage sleep state, also referred to herein as the C6 state. The low-power zero voltage processor sleep state will be described in more detail hereinafter.

A voltage identification (VID) memory 352 that is accessible by the zero voltage entry/exit logic 354 may be included to store a voltage identification code look-up table. The VID memory may be an on-chip or off-chip register or another type of memory, and the VID data may be loaded into the memory via software, basic input/output system (BIOS) code 278 (which may be stored on a firmware hub 279 or in another memory), an operating system, other firmware and/or may be hardcoded, for example. Alternatively, a software look-up table including VID and related data may be otherwise accessible by the logic 350. The VID information may also be stored on the CPU as fuses (e.g., programmable ROMs (PROMs)).

An analog-to-digital converter (ADC) 356 may also be provided as part of the zero voltage entry/exit logic 350 to monitor a voltage supply level and provide an associated digital output as described in more detail below.

Voltage regulator 212 provides a supply operating voltage to the processor 205 and may be in accordance with a version of the Intel Mobile Voltage Positioning (IMVP) specification such as the IMVP-6 specification, for example. For such embodiments, the voltage regulator 212 is coupled to receive VID signals from the processor 205 over a bus 235 and, responsive to the VID signals, provides an associated operating voltage to the processor 205 over a signal line 240. The voltage regulator 212 may include zero voltage sleep logic 302 that is responsive to one or more signals to reduce voltage 240 to the processor 205 to a zero state and then ramp the voltage to the processor back up again after exiting the zero voltage sleep state. For other embodiments, a different type of voltage regulator may be used, including a voltage regulator in accordance with a different specification. Further, for some embodiments, the voltage regulator may be integrated with another component of the system 200 including the processor 205. It should be appreciated that the voltage regulator may or may not be integrated with the CPU dependent upon design considerations.

The memory control hub 215 may include both graphics and memory control capabilities and may alternatively be referred to herein as a graphics and memory control hub (G/MCH) or a North bridge. The graphics and memory control hub 215 and the I/O control hub 225 (which also may be referred to as a South bridge) may be collectively referred to as the chipset. For other embodiments, chipset features may be partitioned in a different manner and/or may be implemented using a different number of integrated circuit chips. For example, for some embodiments, graphics and memory control capabilities may be provided using separate integrated circuit devices.

The I/O control hub 225 of one embodiment includes power management state control logic 242, alternatively referred to herein as C-state control logic. The power management state control logic 242 may control aspects of the transitions between some power management and/or normal operational states associated with the processor 205, either autonomously or in response to operating system or other software or hardware events. For example, for Intel® architecture processors for which at least active mode and power management states referred to as C0, C1, C2 and C4 are supported, the power management state control logic 242 may at least partially control transitions between at least a subset of these states using one or more of a stop clock (STPCLK#), processor sleep (SLP#), deep sleep (DPSLP#), deeper stop (DPRSTP#), and/or stop processor (STPCPU#) signals as described in more detail below.

Also, in one embodiment, voltage from the I/O control hub 225 (V_(I/O) 349) may be provided to the processor 205 in order to provide sufficient power to the dedicated cache memory 340 such that it can store the critical state variables associated with the processor 205 while the rest of the processor 205 is powered down by the reduction of the operating voltage 240 down to a zero state.

For other types of architectures and/or for processors that support different power management and/or normal operational states, the power management state control logic 242 may control transitions between two or more different power management and/or normal operational states using one or more signals that may be similar to or different from the signals shown in FIGS. 2A and 2B.

The mass storage device 230 may include one or more compact disc read-only memory (CD-ROM) drive(s) and associated disc(s), one or more hard drive(s) and associated disk(s) and/or one or more mass storage devices accessible by the computing system 200 over a network. Other types of mass storage devices such as, for example, optical drives and associated media, are within the scope of various embodiments.

For one embodiment, the mass storage device 230 stores an operating system 245 that includes code 250 to support a current and/or a follow-on version of the Advanced Configuration and Power Interface (ACPI) specification. ACPI may be used to control some aspects of power management as described in more detail below. The operating system 245 may be a Windows™ or another type of operating system available from Microsoft Corporation of Redmond, Wash. Alternatively, a different type of operating system such as, for example, a Linux operating system, and/or a different type of operating system-based power management may be used for other embodiments. Further, the power management functions and capabilities described herein as being associated with ACPI may be provided by different software or hardware.

Also, it should be appreciated that system 200 may include a display device, such as a cathode ray tube (CRT) or liquid crystal display (LCD), for displaying information to a user. Further, system 200 may include an alphanumeric input device (e.g., a keyboard), including alphanumeric and other keys, for communicating information and command selections to processor 205. An additional user input device may be cursor control device, such as a mouse, trackball, trackpad, stylus, or cursor direction keys, for communicating direction information and command selections to processor 205, and for controlling cursor movement on the display device.

Another device that may included with system is a hard copy device, which may be used for printing instructions, data, or other information on a medium such as paper, film, or similar types of media. Furthermore, a sound recording and playback device, such as a speaker and/or microphone (not shown) may optionally be included in system 200 for audio interfacing.

Where the system 200 is a mobile or portable system, a battery or battery connector 255 may be included to provide power to operate the system 200 either exclusively or in the absence of another type of power source. Additionally, for some embodiments, an antenna 260 may be included and coupled to the system 200 via, for example, a wireless local area network (WLAN) device 261 to provide for wireless connectivity for the system 200.

(WLAN) device 261 may include a wireless communication module that may employ a Wireless Application Protocol to establish a wireless communication channel. The wireless communication module may implement a wireless networking standard such as Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, IEEE std. 802.11-1999, published 1999.

It should be appreciated that, in one embodiment, the processor 205 of FIGS. 2A and 2B may transition between various known C-states. The normal operational state or active mode for the processor 205 is the C0 state in which the processor actively processes instructions. In the C0 state, the processor 205 is in a high-frequency mode (HFM) in which the voltage/frequency setting may be provided by the maximum voltage/frequency pair.

In order to conserve power and/or reduce thermal load, for example, the processor 205 may be transitioned to a lower power state whenever possible. For example, from the C0 state, in response to firmware, such as microcode, or software, such as the operating system 245, or even ACPI software in some cases, executing a HALT or MWAIT instruction (not shown), the processor 205 may transition to the C1 or Auto-HALT state. In the C1 state, portions of the processor 205 circuitry may be powered down and local clocks may be gated.

The processor may transition into the C2 state, also referred to as the stop grant or SLEEP state, upon assertion of the STPCLK# or similar signal by the I/O controller 225, for example. The I/O controller 225 may assert the STPCLK# signal in response to the operating system 245 determining that a lower power mode may be or should be entered and indicating this via ACPI software 250. In particular, one or more ACPI registers (not shown) may be included in the I/O controller 225 and the ACPI software 250 may write to these registers to control at least some transitions between states. During operation in the C2 state, portions of the processor 205 circuitry may be powered down and internal and external core clocks may be gated. For some embodiments, the processor may transition directly from the C0 state into the C2 state.

Similarly, the processor 205 may transition into the C3 state, also referred to as the Deep Sleep state, in response to the I/O controller 225 or other chipset feature asserting a CPUSLP# signal and then a DPSLP# signal or other similar signals. In the Deep Sleep state, in addition to powering down internal processor circuitry, all phase-lock loops (PLLs) in the processor 205 may be disabled. Further, for some embodiments, a STOP_CPU signal may be asserted by the input/output controller 225 and received by the clock generator 211 to cause the clock generator to halt the clock signal CLK to the CPU 205.

In the system 200 of FIGS. 2A and 2B, a transition into the C4 state or into a zero voltage sleep state may be undertaken in response to ACPI software 250 detecting that there are no pending processor interrupts, for example. ACPI software may do this by causing the ICH 225 to assert one or more power management-related signals such as the exemplary Deeper Stop (DPRSTP#) signal and the exemplary DPSLP# signal. The Deeper Stop (DPRSTP#) signal is provided directly from the chipset to the processor and causes clock/power management logic 350 on the processor to initiate a low frequency mode (LFM). For the low frequency mode, the processor may transition to the minimum or another low operating frequency, for example.

According to some embodiments of the invention, as will be described hereinafter, assertion of the DPRSTP# signal may further cause the internal VID target to be set to a zero voltage level, resulting in a zero operational voltage being applied to the processor 205 by the voltage regulator 212, such that the processor transitions into a very deep sleep state that has very low power consumption characteristics.

According to one embodiment of the invention, an integrated circuit such as processor 205, for example, may initiate a transition to a zero voltage power management state. In one example, processor 205 may be a central processing unit (CPU) 205. Further, the zero voltage management state may be, for example, a deeper sleep state in accordance with ACPI standards. During this transition, the critical state of the CPU 205 may be saved. For example, critical state variables associated with the CPU 205 may be saved in dedicated cache memory (e.g. SRAM) 340.

The operating voltage of the CPU 205 may be subsequently reduced to zero such that the CPU 205 is in a very deep sleep state that has very low power consumption characteristics. Particularly, the voltage regulator 212 utilizing zero voltage sleep state logic 302 may reduce the operating voltage 240 down to zero. As previously discussed, this may be done in conjunction with zero voltage entry/exit logic 354 of clock/power management logic 350 of CPU 205.

In one embodiment, this zero voltage power management state, when implemented in conjunction with ACPI standards, may be referred to as the C6 state.

Subsequently, in response to receiving a request to exit the zero voltage power management state, the CPU 205 exits the zero voltage power management state at a higher reference operating voltage. Particularly, under the control of zero voltage entry/exit logic 354 of CPU 205 and zero voltage sleep logic 302 of voltage regulator 212, as previously described, voltage regulator 212 may raise the reference operating voltage 240 to a suitable level such that the CPU 205 may operate properly. The critical state variables of CPU 205 are then restored from the dedicated cache memory 340.

Thus, the power management scheme allows the CPU 205 to save its state, turn off the power and then wake up when necessary, restore the critical state, and continue where the CPU left off. This may be done, in some embodiments, without explicit support from the operating system 245, and may be accomplished with an extremely short latency period.

More particularly, in one embodiment, in the zero voltage processor sleep state, (which may be referred to as a C6 state in accordance with ACPI standards), the critical state of the CPU 205 is saved in dedicated sleep state SRAM cache 340, which may be powered off the I/O power supply (V_(I/O)) 349, while the core operating voltage 240 for the CPU 205 is taken down to approximately 0 Volts. At this point, the CPU 205 is almost completely powered off and consumes very little power.

Upon an exit event, the CPU 205 indicates to the voltage regulator 212 to ramp the operating voltage 240 back up (e.g. with a VID code 235), relocks the phase lock loop (PLLs) and turns the clocks back on via clock/power management logic 350 and zero voltage entry/exit logic 354. Further, CPU 205 may perform an internal RESET to clear states, and may then restore the state of the CPU 205 from the dedicated sleep state SRAM cache 340, and CPU 205 continues from where it left off in the execution stream. These operations may be done in a very small time period (e.g., approximately 100 microseconds), in CPU 205 hardware, such that it is transparent to the operating system 245 and existing power management software infrastructure.

In one embodiment, this methodology is particularly suited for a CPU 205 having multiple processor cores. In this example, core 320 (e.g. Core #0) and core 322 (e.g. Core #1), i.e. a dual-core CPU, will be discussed as an example. However, it should be appreciated that any suitable number of CPU cores may be utilized. In the dual-core structure, the CPU cores 320 and 322 utilize a shared cache 330. For example, this shared cache 330 may be a level 2 (L2) cache 320 that is shared by the cores 320 and 322.

Further, each core 320 and 322 includes a core ID 321, microcode 323, a shared state 324, and a dedicated state 325. The microcode 323 of the cores 320 and 322 is utilized in performing the save/restore functions of the CPU state and for various data flows in the performance of the zero voltage processor sleep state in conjunction with the zero voltage entry/exit logic 354 of the clock/power management logic 350 of CPU 205. Further, dedicated sleep state SRAM cache 340 is utilized to save the states of the cores, as will be described in more detail below.

It will be appreciated that the system 200 and/or other systems of various embodiments may include other components or elements not shown in FIGS. 2A and 2B and/or not all of the elements shown in FIGS. 2A and 2B may be present in systems of all embodiments.

Turning briefly to FIG. 3, FIG. 3 is a block diagram illustrating an example of dedicated sleep state SRAM cache 340 and an SRAM interface 364, according to one embodiment of the present invention. The dedicated sleep state SRAM cache 340 may store state variables associated with the architectural, micro-architectural, debug state, and microcode patch when CPU 205 is in the zero voltage sleep state (e.g. the C6 state) previously described.

In one example, the size of the SRAM 340 may be 8 KB per CPU core and may be 32 bits wide and may be clocked by the clock/power management logic 350. As previously discussed, the dedicated sleep state SRAM cache 340 may be powered by I/O voltage (V_(I/O) 349) such that its contents are retained when the operating voltage for the CPU 205 is shut off.

The dedicated sleep state SRAM 340 may be structured as 2K entries of 32 bits each and may have ECC protection for single bit error detection and correction. The data path may be 32 bits and support a 2-cycle latency into the array. As can be seen in FIG. 3, the SRAM interface 364 may include a 32 bit data bus from a data buffer 370 which utilizes 32 bit data.

In one example, a control register bus interface may be utilized to interface to the microcode in a simple fashion by utilizing a front end cluster interface to reduce the complexity of addressing the SRAM. The interface may use 2K control registers and a two-level addressing scheme. Two registers may be defined to address the SRAM—the first may be a SRAM base register and the second may be an SRAM data register. Microcode may initialize the base register before starting to access the SRAM. The content of the base register may be used as an index into the SRAM for the next read/write to the data register. After every access to the data register, the index into the SRAM may be auto-incremented by one.

As illustrated in FIG. 3, in one example, the SRAM interface 364 may include a data buffer 370 that buffers 32 bit data into and out of SRAM 340 based upon read/write enablement signals from address decoder 380. Address decoder 380 may also enable a write enable to base register 382 and a reset pointer. Base register 382 may be used to increment register 384 which operates on SRAM 340 by a 12 bit pointer and a 2 bit read/write enable. The content of the base register 382 may be used as an index into the SRAM for the next read/write to the data register. After every access to the data register, the index into the SRAM may be auto-incremented by one. Further, based on a reset pointer, register 384 may reset the SRAM.

Turning now to FIG. 4, FIG. 4 is a flow diagram illustrating a process 400 that may be utilized to enter the zero voltage processor sleep state, according to one embodiment of the present invention. In one embodiment, the following series of operations may be directed by the microcode 323 of the CPU cores 320 and 322 of CPU 205. In the ACPI embodiment, setting forth the C6 state, the entry into the zero voltage processor sleep state may be initiated via an MWAIT instruction, as previously discussed.

From a software point of view, each CPU core 320 or 322 may independently execute the MWAIT instruction. However, in one embodiment, the CPU cores 320 and 322 utilize an L2 shared cache 330 and the same voltage plane. Hence, in this embodiment, there needs to be hardware coordination in the CPU 205 for package level C states, and particularly, the C6 state.

In this embodiment, each core 320 and 322 may execute an MWAIT instruction, and the initializing CPU core goes into a waiting state (e.g. CC6) and waits for the other core to get into the CC6 state as well, before the whole package (e.g. including both cores 320 and 322) may transition into what may be termed the package C6 sleep state.

Looking particularly at FIG. 4, an illustration of entry into the zero voltage processor sleep state is provided. As shown in FIG. 4, each core independently performs a state save when the zero voltage processor sleep state is initiated. Particularly, looking at CPU core #0 320, the first CPU core #0 is active (circle 402) and then a command for a zero voltage sleep state is initiated (e.g. via a sleep or MWAIT instruction) (circle 404). Responsive to this, the state of CPU core 320 is saved at circle 406 to dedicated cache memory 340. This includes the dedicated state 325 and the shared state 324. CPU core 320 then goes into a first sleep state 408 (e.g. CC6) in which it waits for the other core to get into the CC6 state as well, before the whole package can transition into the overall package sleep state (e.g. C6).

In the same manner, the other CPU core (e.g. CPU core #1 322) likewise commands a sleep instruction (e.g. MWAIT) at circle 414 and its state (e.g. both its shared state 324 and dedicated state 325) is also stored to the dedicated cache memory 340 (circle 418). However, in this case, since this is the last core to go into a sleep state, the shared cache 330 is also shrunk and saved to dedicated cache memory 340 (circle 416). Then at circle 420, the second CPU core 322 likewise enters a sleep state (e.g. CC6).

It should be noted that the microcode 323 of the CPU cores 320 and 322 may generally need to know which control registers need to be saved and restored for the zero voltage processor sleep state. The list of the registers may be a subset of the total registers on the CPU 205. For example, the list may be saved as a bit vector (e.g. 1024 bits long). Each bit in the vector may correspond to one control register in the control register address base. For example, microcode may translate the bit position into the control register address and save/restore the register if the bit is “1” and skip it if the bit is a “0”. If a control register requires special handling, the save/restore bit in the vector may be set to “0” and the save/restore handled by special microcodes flow outside of the main save/restore loop.

After microcode operations have been performed, as previously discussed, the zero voltage entry/exit logic 354 of the clock/power management logic 350 takes over the data flows (e.g. C6 flows). Particularly, this occurs after the microcode operations are finished as to state saves (406 and 418) and after each CPU core 320 and 322 has reached an individual sleep state 408 and 420 (e.g. CC6 state).

At this point, all the required state of the CPU 205 has been saved or flushed from the CPU 205. The zero voltage entry/exit logic 354 of the clock/power management logic 350 then initiates an external platform level entry sequence (e.g. a C6) sequence by performing an I/O register read from the ICH 225. In one embodiment, this may be the ACPI defined method of entering CPU “C” states.

The sequence of events from the external bus perspective from this point is shown in FIG. 4. The I/O commands 410 may be issued from the ICH 225/MCH 215. Particularly, a stop clock signal may be asserted at circle 430 (e.g. STPCLK#). Then, a sleep signal may be asserted. (circle 431) (e.g. SLP#). Further, at circle 432, a deep sleep signal may be asserted (DPSLP#). These commands are issued in the previously-described order such that the CPU 205 responds by shutting off its internal clock distribution and then the PLLs.

When at circle 435, a deeper stop signal is asserted (e.g. DPRSTP#), the CPU 205 switches its VID to a zero voltage level in order to tell the voltage regulator 212 to remove power and that it is safe to do so. This may be referred to as a C6 VID. In this way, power is deasserted. However, it should be appreciated that instead of zero volts, an otherwise very small amount of voltage may be chosen as the VID.

It should be appreciated that instead of an exact zero voltage level, the voltage level may set to an “approximately zero voltage level.” This approximately zero voltage level may be a very low voltage level, such as 0.3V or 0.5V. In some embodiments, such a very low approximately zero voltage level may optimize entry and exit latencies to and from the sleep state, respectively. Additionally, it should be appreciated, that the approximately zero voltage level may chosen for the system (e.g. in silicon) during manufacturing (e.g. after a tape-out) and may be programmed differently during different steppings and revisions of the CPU.

The package of CPU cores (e.g. core 320 and core 322) is considered to be in a package sleep state (C6) at circle 440 when the operating voltage 240 from voltage regulator 212 reaches zero volts or another nominally small level. It should be noted that since there is no active device pulling down the operating voltage, it simply drifts down slowly as the charge leaks away due to the CPU 205 leakage. Thus, the CPU 205 has entered the zero voltage package sleep state (C6). It should be appreciated that the sequence of operations previously described may be effectuated in a variety of different orders, and that the previously described order of operations is just one example.

Turning now to FIG. 5, FIG. 5 is a flow diagram illustrating an example of a process 500 for an exit sequence from the zero voltage processor sleep state. Typically, an exit out of the zero voltage processor sleep state begins when the chipset detects an event that necessitates waking up the CPU 205—most likely an interrupt event. However, it should be noted that the chipset may proceed with master accesses to memory without waking up the CPU when doing snoops. The sequence of external events and handshakes that occur between the chipset and the CPU 205 during an exit from the zero voltage processor sleep state will be discussed with reference to FIG. 5. Particularly, this sequence may be considered the reverse of what happens during the previously-described entry phase.

From the package sleep state (C6) (circle 440) in one embodiment, the deeper stop signal (DPRSTP#) is deasserted (circle 502) which is detected by the CPU 205 and the zero voltage entry/exit logic 354 of the clock/power management logic 350 such that the low frequency mode (LFM) VID is sent to the voltage regulator 212. This indicates to the voltage regulator 212 to drive the core operational voltage back up to the required VID.

At a predetermined time (e.g. controlled by timers in the ICH 225), a signal to assert the clocks back on is asserted and the deep sleep (DPSLP#) signal is de-asserted (circle 505), which initiates the PLLs of the clock/power management logic 350. After this, CPU 205 initiates an internal RESET (circle 506). After this reset is complete, CPU 205 has the power and clocks engaged and is ready to restore the critical state variables associated with the CPU 205.

As an example, in the ACPI embodiment, typically during a C-state exit event, CPU 205 waits until the STPCLK# deassertion to do anything internally within the CPU. However, according to embodiments of the present invention, in the zero voltage processor sleep state (e.g. C6) due to the longer latency of restoring the states, etc., STPCLK# is overridden and a state restore (circle 510 and circle 530) for the respective cores 320 and 322 of the CPU 205 is begun in preparation of the C6 exit as soon as the power and clocks are available. Once the states of the cores 320 and 322 are restored, CPU 205 is ready to continue from where it stopped. Microcode 323 from both of the cores 320 and 322 of the CPU 205 are reset (circles 512 and 532), respectively.

However, neither of the CPU cores 320 and 322 become active (circles 514 and 534), respectively, and being executing instructions, until the sleep signal is de-asserted (circle 540) and the stop clock signal (STPCLK#) is de-asserted. However, once the stop clock is de-asserted and the sleep signal is deasserted, both the CPU 205 and cores 320 and 322 and the chipset are all powered on and are operational, and both cores become active (circle 514 and circle 534). CPU 205 then typically begins some code fetches since its caches are empty and then will begin normal operations. More particularly, both cores are woken up in the C0 state. The operating system may determine which CPU core will handle events and the other core may be put back into a sleep state soon thereafter.

Advantageously, as explained earlier, because CPU 205 does the majority of C-state coordination in hardware, software can independently request to enter the zero voltage processor sleep state (e.g. C6) in each core without any software coordination with the other core. More particularly, by utilizing the zero voltage processor sleep state (e.g. C6) the power required by the CPU 205 in the sleep state is reduced virtually to zero.

Also, in the ACPI embodiment, in which the C6 state is described, an efficient method to achieve the state save and the restoration of the shared state 324 of the cores 320 and 322 of processor 205 using a core ID field 321 is also disclosed. This method serves as the synchronization for the core exit into C0. The core identifiers 321 may be hardwired into each core 320 and 322 via fuses and may be available at all times to the cores of the CPU 205.

Both the cores 320 and 322 may save the dedicated state 325 and the shared state 324 at the time of entry. The cores 320 and 322 preferably will not use any ordering method (like a semaphore) during the state save. At the end of the state save, each core may record its unique core identifier 321 in a hardware state.

When the second core completes its state save, that core may write its unique core identifier 321 in the same hardware state, effectively overriding the core identifier of the first core. This hardware state that contains the core identifier will be saved during the C6 residence by powering it with an always ON power source. At C6 exit, as previously discussed, when the cores 320 and 322 perform state restore, the core identifier that matches the stored core identifier will be allowed to perform the restore of the shared state 324. Both cores 320 and 322 may perform the restore of the dedicated state 325. The cores will then access the stored core identifier 321 to check whether they need to perform the restore of the stored state. Merely one core needs to be allowed to perform the restore. The other cores may wait while the shared state 324 is being restored. When the restore is complete, both the cores proceed to the C0 state.

In one embodiment, the cores 320 and 322 may use a hardware semaphore “C6 order semaphore” to ensure exclusive access to the core identifier 321 field during restore. An example of pseudo-code to enable the shared state restore is provided below:

Core that went into CC6 first Restore dedicated state Grab C6 Order Semaphore Read C6 wakeup.coreID Match C6 wakeup.coreID against own coreID No match, release C6 Order Semaphore Restore_Complete: Wait Read C6 wakeup.coreID If coreID !=00 jump to Restore_Complete If coreID = 0 jump to instruction after mwait Core that went into CC6 last Restore dedicated state Grab C6 Order Semaphore Read C6 wakeup.coreID Match C6 wakeup.core ID against own coreID Match, lock C6 Order Semaphore Go to shared state restore

In another embodiment of the invention, during the transition to the zero voltage power management state for the processor, the operational voltage applied to the processor by the voltage regulator may be reduced to approximately zero while an external voltage is continuously applied to a portion of the processor that saves the state variables for the processor. For example, the processor may include a voltage plane that applies the external voltage to the portion of the processor to save the state variables that is separate from the operational voltage plane that applies the operational voltage to the processor.

In particular, a system architecture may be provided that allows a processor to enter the zero voltage power management state without utilizing a dedicated cache memory to store the critical state of the processor during the transition to the zero voltage power management state. This may be achieved by utilizing a voltage plane to the processor core that always remains powered. More particularly, an external voltage from an external voltage source (e.g. from the ICH) is continuously applied via the voltage plane to a portion of the processor that saves the state variables of the processor throughout the zero voltage power management state. The portion of the processor that saves the state variables may include model-specific registers (MSRs) and control registers (CRs) of the processor core. In particular, in this scheme, transistors related to model-specific registers (MSRs) and control registers (CRs) of the processor core are placed on the voltage plane which always remains powered.

With reference to FIGS. 6A and 6B, FIGS. 6A and 6B are block diagram diagrams of an exemplary system 600 that may implement the zero voltage power management state transition approach, previously described, but with a key difference, in that a voltage plane 605 is provided to critical regions of processor cores 320 and 322 of processor 604 that always remains powered to preserve the critical state of the processor cores during the zero voltage power management state. It should be noted that FIGS. 6A and 6B is identical to the previously-described system of FIGS. 2A and 2B with the exception of a few differences, to be hereinafter described in detail.

In one embodiment, this methodology is particularly suited for a CPU 604 having multiple processor cores. In this example, core 320 (e.g. core #0) and core 322 (e.g. core #1) (i.e. a dual-core CPU), will be discussed as an example. However, it should be appreciated that any suitable number of CPU cores may be utilized. Further, as previously described, in the dual-core structure, the CPU cores 320 and 322 utilize a shared cache 330. For example, this shared cache 330 may be a level 2 (L2) cache 320 that is shared by the cores 320 and 322.

According to one embodiment of the invention, an integrated circuit, such as CPU 604, may initiate a transition to a zero voltage power management state. Further, the zero voltage management state may be, for example, a deeper sleep state in accordance with ACPI standards. During this transition, the critical state of the CPU 604 is continuously kept powered on via voltage plane 605 with power from an external voltage source.

The operating voltage of CPU 604 may be reduced to zero such that CPU 604 is in a very deep sleep state that has very low power consumption characteristics. Particularly, voltage regulator 212 utilizing zero voltage sleep state logic 302 may reduce operating voltage 240 down to zero. As previously described, this may be done in conjunction with zero voltage entry/exit logic 354 of clock/power management logic 350 of the CPU 604. In one embodiment, this zero voltage power management state, when implemented in conjunction with ACPI standards, may be referred to as the C6 state, as previously described.

However, as opposed to the embodiment previously described with reference to FIGS. 2A and 2B, CPU 604 of system 600 does not include a dedicated cache memory (e.g. SRAM 340 as in FIGS. 2A and 2B). This is because voltage plane 605 is provided that continuously provides power to the model-specific registers (MSRs) and control registers (CRs) of the critical portions of processor cores 320 and 322 such that these portions of the processor cores continuously remain powered during the zero voltage power management state (e.g. C6 state). Additionally, because save/restore functions for CPU 604 during the entrance and exit of the zero voltage processor sleep state are not required microcode sections to enable this functionality are not required (e.g. microcode 322 and 323 of FIGS. 2A and 2B).

Thus, in one embodiment, voltage plane 605 continually provides power to the critical model-specific and control registers for processor cores (#0 and #1) 320 and 322 including registers for the core IDs 321, shared states 324, and dedicated states 325, respectively. In particular, as shown in FIG. 6A, voltage plane 605 includes a voltage line 610 to core 320 and a voltage line 612 to core 322.

However, with respect to the other structural and functional aspects of CPU 604 and the zero voltage power management state previously described with reference to FIGS. 2A and 2B, the structure and functionality is substantially the same, and will not be repeated for brevity's sake.

All of the necessary state information for processor cores 320 and 322 needed to resume CPU operations after entry into and exit from the zero voltage power management state is available. This is because the critical portions of processor cores 320 and 322 remain continuously powered during the zero voltage power management state by voltage plane 605.

In one embodiment, voltage plane 605 is continuously powered by the I/O power supply from the I/O control hub 225 shown as V_(I/O) 349. Since CPU 604 includes an always-on voltage plane 605 powering the critical portions of the processor cores 320 and 322, the critical states of the processor cores 320 and 322 of CPU 604 do not have to be saved and restored or “shadowed” to other structures of the CPU (e.g. such as a dedicated cache memory (SRAM)) because, by design, the critical states of processor cores 320 and 322 are never lost because of continuous power being applied by the always-on voltage plane 605. In particular, in one embodiment, the core ID 321, shared state 324, and dedicated state 325 of the processor cores, respectively, are the critical portions that remain powered on by voltage plane 605 during the zero voltage power management state. This voltage plane is maintained irrespective of the operational voltage 240 associated with the power plane for entering and exiting the zero voltage power management state (e.g., C6) provided by the voltage regulator 212.

In this embodiment, CPU 604 resumes execution from where it left off before entering the zero voltage management power state by re-powering the zero voltage power management plane (e.g. C6) via operational voltage 240, resetting the transistors in this power plane, and restarting execution based upon the state of the critical state of processor cores 320 and 322 that have continuously remained powered by the always-on voltage plane 605 during the zero voltage management power state. It should be noted that because the number of transistors on the zero voltage power management state (C6) power plane is significantly larger than the number of transistors used to implement the critical-state registers along voltage plane 605 for processor cores 320 and 322 that significant power savings are still achieved.

Thus, according to one embodiment of the invention, CPU 604 may initiate a transition to a zero voltage power management state. During this transition, the critical state of the CPU 604 is continuously kept powered via voltage plane 605. The operating voltage of CPU 604 may be reduced to zero such that CPU 604 is in a very deep sleep state that has very low power consumption characteristics. Particularly, voltage regulator 212 utilizing zero voltage sleep state logic 302 may reduce operating voltage 240 down to zero (e.g., the C6 state). As previously described, this may be done in conjunction with zero voltage entry/exit logic 354 of clock/power management logic 350 of the CPU 604.

Subsequently, in response to receiving a request to exit the zero voltage power management state, CPU 604 exits the zero voltage power management state at a higher reference operating voltage. Particularly, under the control of zero voltage entry/exit logic 354 of CPU 604 and zero voltage sleep logic 302 of voltage regulator 212, as previously described, voltage regulator 212 may raise the reference operating voltage 240 to a suitable level such that CPU 604 may operate properly. The critical state variables of CPU 604 are already present as they have been continually powered throughout the zero voltage power management state by voltage plane 605.

In particular, upon an exit event, CPU 604 indicates to voltage regulator 212 to ramp the operating voltage 240 back up (e.g. with a VID code 235), re-locks the phase lock loops (PLLs), and turns the clock back on via clock/power management logic 350 and zero voltage entry/exit logic 354. CPU 604 then continues from where it left off in the execution stream. These operations may be done in a very small time frame in CPU 604 hardware such that it is transparent to the operating system 245 and existing power management software infrastructure.

Thus, this power management scheme allows CPU 604 to be powered-off and then wake up when necessary and continue where it left off. This may be done, in some embodiments, without explicit support from the operating system 245, and may be accomplished with almost no latency because there is no time needed to save or restore processors states. Moreover, as previously described, the core operating voltage 240 for CPU 604 may be taken down to approximately 0 Volts. At this point, CPU 604 is almost completely powered off and consumes very little power.

As has been described, this methodology is particularly suited for a CPU 604 having multiple processor cores, according to one embodiment. In this example, core 320 (e.g. core #0) and core 322 (e.g. core #1) (i.e. a dual-core CPU), will be discussed as an example. Each core 320 and 322 may include a core ID 321, a shared state 324, and a dedicated state 325 along with other information. However, the core ID 321, shared state 324, and dedicated state 325 are critical sections that remain powered on by voltage plane 605 during the zero voltage power management state. Further, as previously described, in the dual-core structure, the CPU cores 320 and 322 utilize a shared cache 330. For example, this shared cache 330 may be a level 2 (L2) cache 320 that is shared by the cores 320 and 322.

Turning now to FIG. 7, FIG. 7 is a flow diagram of a process 700 for an integrated circuit device, such as a processor (e.g. CPU) to enter and exit voltage power management state. The processor first initiates a transition to a zero voltage power management state (block 705).

During the zero voltage power management state, a separate voltage source is always applied to a portion of the processor to save state variables of the processor (block 710). In particular, a separate external power source is applied in a selective fashion to the processor core to save the critical state of the processor core. As previously described, a voltage plane is provided that continuously provides power to the model-specific registers (MSRs) and control registers (CRs) of the critical portions of processor cores such that these portions of the processor cores continuously remain powered during the zero voltage power management state (e.g. C6 state). These selected critical model-specific and control registers for processor cores (#0 and #1) 320 and 322 may include registers for the core IDs 321, shared states 324, and dedicated states 325, respectively.

Further, as previously described, in the zero voltage power management state, the operating voltage of the processor is reduced to approximately zero (block 715). In this way, the processor is in a very deep sleep state (e.g. C6 state) that has very low power consumption characteristics.

Subsequently, in response to receiving a request to exit the zero voltage power management state, the processor exits the zero voltage power management state at a higher reference operating voltage (block 720).

In this embodiment, because the critical state of the processor core remains powered-on by the always-on voltage plane, the critical state is automatically restored and the processor can continue where it left-off before it entered the zero voltage power management state.

It should be noted that the process for two CPU cores entering the zero voltage management power state utilizing an always-on voltage plane is almost exactly the same as the previously-described process 400 with reference to FIG. 4 and the description applies equally. Therefore, it will not be repeated for brevity's sake. The only difference being that process functions 406 and 418 related to the states of the processor cores being saved are not utilized. This is because a dedicated cache memory is not utilized in this embodiment as the critical state of the processor cores are maintained in an always-on state via the voltage plane, as previously described.

Further, it should be noted that the process for two CPU cores exiting the zero voltage management power state utilizing an always-on voltage plane is almost exactly the same as the previously-described process 500 with reference to FIG. 5 and the description applies equally. Therefore, it will not be repeated for brevity's sake. The only difference being that operations 510 and 530 are not required because there is no need to restore the state to the processor cores from the dedicated cache. This is because a dedicated cache memory is not utilized in this embodiment as the critical state of the processor cores are maintained in an always-on state via the voltage plane, as previously described. Thus, upon exit from the zero voltage power management state the processor wakes up with the critical state already powered on such that the processor cores can automatically be active. Additionally, there is no need for the microcode reset (operations 512 and 532) to restore the states.

In the previously-described embodiments of FIGS. 4, 5, 6, and 7, it should be noted that save/restore steps 416 and 418 of FIGS. 4 and 510 and 530 of FIG. 5 are not required such that there virtually no latency to enter or exit a low power state because no time is needed to save or restore the processors states.

With reference to FIG. 8, in an alternative embodiment, additional system logic 800 may be provided to a system, such as system 600 of FIGS. 6A and 6B, wherein, instead of merely utilizing a voltage plane 605 to power processor cores additional logic to switch contexts between process cores is also provided.

For example, system logic 800 may include a context logic circuit 802 and a context router 820. In particular, context logic circuit 802 and context router 820 in addition to providing power to processor cores by providing a voltage plane along voltage/signal lines 822 and 824, respectively, also provides for the transfer of digital signal information along voltage/signal lines 822 and 824 such that the contexts of various cores within a CPU, and even between multiple CPUs, may be swapped.

For example, in one embodiment, instead of utilizing voltage plane 605 in CPU 604 of FIGS. 6A and 6B, system logic 800 that includes context logic circuit 802, context router 822, and a first voltage/signal line 822 to processor cores 320 and 322 may be utilized.

Additionally, as particularly shown in FIG. 8, system logic 800 may be utilized to not only enable context switching between multiple processor cores of a single CPU but between multiple processor cores of multiple CPUs. This more generalized multiple CPU system will be discussed hereinafter. However, as will be appreciated by those of skill in this art, this description equally applies to the single CPU case having multiple processor cores as in CPU 604 of FIGS. 6A and 6B.

By utilizing system logic 800, not only do all of the processor cores of multiple CPUs remain powered, but the context of each of the processor cores may be routed to another processor core of the same or a different CPU upon exit of the zero voltage power management state (e.g. C6 state).

As illustrated in FIG. 8, context logic circuit 802 may be powered by voltage (e.g. V_(I/O) 349) from the I/O control hub and may control the routing of different contexts (e.g. context X 810 and context Y 812) in addition to the previously-described provision of the always-on voltage to the critical sections of the CPU cores.

In this example, a first CPU A having multiple cores (0,1 . . . N) 830, 832 and 834 and a second CPU B having multiple cores (0,1 . . . N) 840, 842, and 844 is illustrated. Utilizing context router 820, context X 810 and context Y 812, for example, may be routed to any one of the processor cores of the multiple CPUs (CPU A and B) upon exit from the zero voltage management power state via signal lines 822 and 824, respectively. In this way, a very flexible mechanism is provided as to how contexts may be restored to the processor cores of CPUs in a computer system. In particular, based upon core balancing considerations, context logic circuit 802 may restore a context to any suitable processor core within the same CPU or in a different CPU in order to optimize efficiency.

It will be appreciated that, while exemplary embodiments have been described in reference to zero voltage processor sleep state that transitions into and out of a deeper sleep C6 state associated with a central processing unit and voltage regulator, that various embodiments may be applied to different types of power management configurations and/or for different types of integrated circuits. Further, although voltage control is accomplished using VID signals for the exemplary embodiments described herein, other approaches to providing voltage control are within the scope of various embodiments.

Thus, various embodiments of a method and apparatus for a zero voltage process sleep state are described. In the foregoing specification, embodiments of the invention have been described with reference to specific exemplary embodiments thereof. It will, however, be appreciated that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. Thus, the specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 

What is claimed is:
 1. A processor comprising: a first processor core having first registers; a second processor core having second registers; first logic to transition the first processor core to a zero voltage power management state, wherein an operational voltage at least for the first processor core is to be reduced to approximately zero volts and a voltage to be received from an external voltage source is to be applied to the first registers to save state variables of the first processor core and to the second registers to save state variables of the second processor core during the zero voltage power management state; and second logic to switch operating contexts between the first and second processor cores after exit from the zero voltage power management state.
 2. The processor of claim 1, further comprising a first voltage plane to apply the operational voltage to the first processor core and a second voltage plane different from the first voltage plane to apply the voltage to the first registers of the first processor core to save the state variables.
 3. The processor of claim 1, wherein the voltage is to be continuously applied to the first registers to save the state variables of the first processor core throughout the zero voltage power management state.
 4. The processor of claim 1, wherein the first registers include model-specific registers.
 5. The processor of claim 1, wherein the first registers include control registers.
 6. The processor of claim 1, wherein the external voltage source comprises a voltage from an input/output (TO) controller.
 7. The processor of claim 1, further comprising a cache memory coupled to the first and second processor cores, wherein the cache memory is to be powered down in the zero voltage power management state.
 8. The processor of claim 1, wherein the first state variables are to be available after exit from the zero voltage power management state without a restore operation.
 9. A system comprising: a processor having at least a first processor core with first state storage and a second processor core with second state storage; a voltage regulator coupled to the processor to provide an operational voltage to the processor, wherein the voltage regulator is operable to reduce the operational voltage to the processor to approximately zero volts for a transition to a zero voltage power management state for the processor; a controller coupled to the processor, the controller to provide a voltage to the first state storage and the second state storage to save state information of the processor during the zero voltage power management state; and logic to switch operating contexts between the first processor core and the second processor core after exit from the zero voltage power management state.
 10. The system of claim 9, wherein the processor comprises a first voltage plane to apply the voltage to the first state storage and the second state storage to save the state information, wherein the first voltage plane is separate from a second voltage plane that is to apply the operational voltage to the processor.
 11. The system of claim 9, wherein the controller is to continuously apply the voltage to the first state storage and the second state storage to save the state information throughout the zero voltage power management state.
 12. The system of claim 9, wherein the controller comprises an input-output controller.
 13. The system of claim 9, wherein the first state storage includes control registers of the first processor core.
 14. The system of claim 9, wherein the first state storage includes model-specific registers of the first processor core.
 15. A non-transitory machine-readable storage medium having stored thereon instructions, which when executed by a machine, cause the machine to perform operations comprising: transitioning a processor to a zero voltage power management state, wherein an operational voltage applied to the processor is reduced to approximately zero volts; applying a voltage to a first state storage of a first processor core of the processor and a second state storage of a second processor core of the processor to save state variables of the processor during the zero voltage power management state; exiting the zero voltage power management state at an operational voltage higher than the approximately zero volts; and switching operating contexts between the first processor core and the second processor core after exiting from the zero voltage power management state.
 16. The non-transitory machine-readable storage medium of claim 15, further comprising instructions to continuously apply the voltage to the first state storage and the second state storage to save the state variables throughout the zero voltage power management state. 